1. Field of the Invention
The present invention relates to a lithography apparatus which uses a plurality of charged-particle beams, and a method of manufacturing a device.
2. Description of the Related Art
When an electron-beam exposure apparatus is used to draw the device pattern of the (n+1)th layer on the device pattern of the nth layer formed on the wafer, the device patterns of the nth and (n+1)th layers are aligned. For the alignment (overlay), wafer alignment measurement is performed. In the wafer alignment measurement, the positions of a plurality of alignment marks already formed on the wafer by exposure are measured using an off-axis alignment scope, and the positions of all shots formed on the wafer by exposure are obtained based on the measured values. In this way, the position of each shot formed in the nth layer on the wafer is obtained, and thereupon this shot is moved to the electron-beam exposure position to draw by overlay exposure the device pattern of the (n+1)th layer on the pattern drawn in the nth layer. At this time, positioning of each shot is controlled based on the length of a baseline which connects the position of the alignment mark measured by the alignment scope and the electron-beam exposure position. Positioning of each shot is also controlled based on global alignment measurement values.
A baseline is a line (line segment) which connects the optical axis of the alignment scope and the electron-beam exposure position, indicates their relative positional relationship, and can be represented by a two-dimensional vector quantity. Baseline measurement is performed in accordance with the following procedure.
(1): The position of a reference mark formed on a reference mark table on a wafer stage is measured using the alignment scope.
(2): The position of the reference mark formed on the reference mark table on the wafer stage is measured using the electron beam.
(3): A baseline that is the difference (relative positional relationship) between the position of the alignment mark on the wafer measured using the alignment scope and that of the alignment mark on the wafer measured using the electron beam is calculated based on the difference between the positions of the wafer stage in measurement which uses the alignment scope and the electron beam, respectively.
Japanese Patent Laid-Open No. 2000-133566 proposes a technique of measuring a baseline in an electron-beam exposure apparatus having a single electron beam (to be simply referred to as a single beam hereinafter).
A multibeam electron-beam exposure apparatus which uses a large number of electron beams to improve the throughput has been proposed. When an electron beam is used for baseline measurement in a multibeam electron-beam exposure apparatus, as in a single-beam electron-beam exposure apparatus, it is necessary to select an electron beam, to be used to detect the position of the reference mark, from a plurality of electron beams. The inventor of the present invention found that depending on which one of a plurality of electron beams is used to detect the position of the reference mark, the incident angle of the electron beam on the reference mark changes, so the detection precision of the position of the reference mark and consequently the baseline measurement precision also change. This holds true not only for an exposure apparatus which uses a plurality of electron beams but also for a general lithography apparatus which uses a plurality of charged-particle beams.